The present invention relates generally to signal converters and, in particular, to determining a conversion characteristic of a converter element in a signal converter.
As is known in the art, a signal converter such as a sigma-delta modulator converts an analog input signal to a digital output signal. Typically, the sigma-delta modulator includes an input, a primary path, a feedback path, and an output. Along the primary path of the sigma-delta modulator are one or more arithmetic devices, a noise-shaping filter, and a primary analog-to-digital converter. The primary analog-to-digital converter employs a sampling clock having a frequency greater than the bandwidth of the analog input signal, per an oversampling ratio. Along the feedback path of the analog-to-digital converter are one or more feedback digital-to-analog converters.
Improvement in the resolution of the digital output signal of the sigma-delta modulator is significantly limited by the degree of non-linearity in the feedback digital-to-analog converter. Non-linearity in the feedback digital-to-analog converter commonly arises from imperfection in one or more converter elements which comprise the feedback digital-to-analog converter. In addition, changes in environmental conditions can adversely affect accuracy of the digital-to-analog signal conversion by the converter elements.
In one example of a sigma-delta modulator, there is no measurement of actual performance of digital-to-analog signal conversion by the elements of the feedback digital-to-analog converter. This design suffers a shortcoming of not accounting for imperfection and/or mismatch among the converter elements. A further shortcoming of this design is the failure to account for environmental conditions which adversely affect performance of the sigma-delta modulator.
Another example of a sigma-delta modulator employs a technique of noise-shaped dynamic element matching. The technique involves using a different subset of converter elements in the feedback digital-to-analog converter for each digital code to be converted to analog form. Each subset of converter elements is chosen so that most of the noise spectrum generated by the inaccurate elements is outside the band of the analog input signal. One shortcoming of this design is that the achievable linearity is limited by inaccuracy in the original matching of converter elements of the feedback digital-to-analog converter. A further shortcoming is the requirement of a substantial amount of circuitry between the output from the primary analog-to-digital converter and the input to the feedback digital-to-analog converter. The additional circuitry is required to switch among the subsets of converter elements corresponding to each digital code to be convened. The amount of circuitry required increases substantially with the number of elements in the feedback path. The addition of circuitry undesirably causes delay in the feedback path. This additional delay adversely affects the stability of continuous-time sigma-delta modulators and ultimately limits the sampling rate of either continuous- or discrete-time sigma-delta modulators. As yet another shortcoming, the technique is less effective if the oversampling ratio of the primary analog-to-digital converter is less than thirty-two, thereby undesirably introducing a limit on the overall bandwidth of the sigma-delta modulator in view of the cost of providing an increase in the frequency of the sampling clock.
A further example of a sigma-delta modulator temporarily halts the basic conversion of the input signal to the output signal in the sigma-delta modulator to perform a calibration of the converter elements of the feedback digital-to-analog converter. A shortcoming of this design is the requirement of interrupting the basic operation by the sigma-delta modulator of converting the input analog signal to the output digital signal whenever calibration of any converter element of the feedback digital-to-analog converter takes place. Another shortcoming is the failure to correct for environmental changes arising at times between calibrations of the converter elements of the feedback digital-to-analog converter.
Thus, a need exists for determination of a conversion characteristic of a converter element contemporaneous with conversion of an input signal to an output signal converter.